1. Field of the Invention
The present invention relates to the preparation of non-stick (non-agglomerating) particles based on a crosslinked organopolysiloxane composition, especially elastomer particulates produced by polyaddition crosslinking, which particulates may contain an active principle encapsulated by and/or dispersed within the elastomer.
2. Description of the Prior Art
There is an increasing demand in this art for silicone elastomer particles with a view to their use in two principal types of applications:
(a) incorporation of these particles as filler material into a polymer matrix, in particular into a crosslinkable organopolysiloxane composition to produce an elastomer, with a view especially to an enhancement of flexibility, and
(b) conditioning (encapsulation or dispersion) of an active principle by these particles, with a view to the protection and ultimately controlled release thereof.
Among the numerous patents describing processes for the preparation of silicone elastomer particles or microparticles crosslinked by polyaddition and/or polycondensation reactions, the following are particularly representative:
EP-A-252,510, which describes a process comprising mixing the various constituents of a liquid silicone elastomer, which may contain a siliceous filler, at a temperature ranging from -60.degree. C. to +5.degree. C. and then atomizing the mixture in air heated to a temperature of from +80.degree. C. to 200.degree. C., in order to produce particles of substantially spherical shape and of a particle size ranging from a few tens of millimicrometers to several hundreds of micrometers;
U.S. Pat. No. 4,248,751, describing a process for the preparation of a silicone latex of polyaddition silicone elastomer particles dispersed in an aqueous phase, which entails polymerizing, in aqueous emulsion, a mixture of vinylated silicone oils, hydrogen-containing silicone oils and a surfactant, and then crosslinking such oils by heating after addition of a platinum catalyst (it is possible to add a colloidal silica to the latex in order to improve the properties of the film-forming elastomer covering obtained after coating of the latex onto a support and evaporation of the water, but this patent neither describes nor suggests the preparation of silicone elastomer particles having a particle size greater than 50 .mu.m from the latex emulsion);
EP-A-217,257, describing a process for the preparation of silicone particles of a particle size of from 0.01 to 10 mm, including emulsion-polymerizing a diorganocyclopolysiloxane bearing vinyl and mercaptoalkyl groups in the presence of an alkylarylsulfonic acid, breaking the emulsion by adding magnesium sulfate thereto and by heating, introducing a vinyl monomer and effecting a graft polymerization in the presence of a free radical initiator;
U.S. Pat. No. 4,594,134 which describes the preparation of microparticles or spherical particles, of a particle size ranging from 2 to 300 .mu.m, of silicone polyaddition elastomer, produced by atomization of a platinum-catalyzed composition, including an inhibitor of the platinum, in a dryer at about 230.degree. C. The crosslinking can be effected without platinum and under UV irradiation, but with a photosensitizer. Prior to crosslinking, the composition can be dispersed in a liquid medium such as an organic solvent or water;
Japanese Kokai 87/257,939, describing the preparation of a silicone powder by drying and atomization of an aqueous emulsion of a polyaddition silicone composition;
Ep-A-267,003, which describes the preparation of microspheres of silicone elastomer by dispersing, in water in the presence of a surfactant, an organopolysiloxane composition which is crosslinkable by addition reactions of the MICHAEL type, and crosslinking of the composition;
U.S. Pat. No. 4,370,160, describing a process similar to EP-A-267,003, except that the organopolysiloxane composition is crosslinked under UV irradiation in the presence of a photosensitizer.